Injection devices of a general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
Often, such injection devices are required to work with sealed hypodermic syringes which typically have a hermetically sealed cover or “boot” that covers the hypodermic needle and maintains the sterility of the syringe contents. Naturally, it is necessary to maintain the sterility of the syringe contents up to the point of administration, which means that for devices that are designed to be disposable, the boot must be removed with the syringe inside the injection device.
In previously known injection devices, the action required to remove the boot from the syringe entails either pulling the boot away from the syringe or twisting the boot and, at the same time, pulling it away from the syringe.
If the injection devices are used by patients having rheumatoid arthritis or elderly or weak patients, it is difficult for the patient to remove the boot from the syringe prior to its use. In addition, the boots are small and fiddly to grip for all users but particularly for users who have poor flexibility in their joints or limited manual dexterity. Since the boot of the syringe may be positioned inside the housing, there may also be limited access to the boot.
Co-pending United Kingdom patent application no. 0412051.5, the contents of which is incorporated herein by reference, describes a cap for an injection device that connects to the boot of the syringe so that removal of the housing closure member from the housing causes removal of the boot from the syringe.
In certain types of syringe, for example a Bünder ReadyJect™ type syringe, the boot must be twisted to break a frangible joint before it can be pulled off. An inexperienced user of the injection device might attempt to pull the cap of the injection device off without first twisting. This could lead to either the boot pulling out of the cap of the injection device and remaining on the syringe or the syringe components separating. In either case, the user will not be able to use the injection device to successfully deliver a drug product.